Core making generally involves mixing sand with a binder that sets by chemical action (cold cure method) or by heat (hot-box method). The cold cure processes have proved commercially successful because of their speed and core strength achievable without the need for thermal energy. However, the noxious and contaminating characteristics of the curing gas for cold cure resins have been a considerable draw back. With respect to the hot box type, one process stands out in this regard because the chemicals employed are relatively free from noxious odors and the chemical system is responsive to thermal energy which can be generated by microwaves. This hot box system employs a water soluble inorganic binder, such as sodium and potassium silicates, which while it can be cured either by CO.sub.2 gas or esters, it can also be cured by heat.
The concept of using microwave energy to cure sand mixtures for core making is known. However, in spite of the general state of the art with respect to microwave technology, several problems remain to be solved with respect to making better quality cores which can be employed in aluminum semi-permanent molding. These problems fall into basically two categories, (1) the rapidity of heating with microwave energy causes uncontrolled rapid expansion of gases within the sand core leading to cracking, (2) uneven heating and curing of the core sand mixture may result from unusual core configurations and from reliance solely on molecular friction of the resin to do the heat curing, and (3) microwaved sand cores have an unusual tendency to absorb moisture during storage which eventually destroys their desired strength characteristics, and (4) lack of a uniform gradient of curing to insure proper strength and at the same time provide easy shakeout of the sand grains. Accordingly, it is important that there be improved molecular polar heating of the water-resin solution within the sand mixture without experiencing extremely rapid vapor generation and with improved heat transfer from the surrounding core box to insure a uniformity of curing gradient, and that there be some mechanism to reduce hygroscopic tendencies of the core.